Sound reproduction devices with a digital audio signal processor connected downstream of the audio signal source are well known in the state of the art, so that no further details need be provided here. If the audio signal source is one that contains two separate audio signal channels for the stereophonic transmission of sound events, and if a fader control is connected downstream of the audio signal source, which distributes the audio output signals in the sound channels to two different output channels of the fader control, the state of the art recognizes two solutions which make the stereophonic audio signals accessible to digital processing.
Before providing further details, the function of a fader control will be explained. In a very simple configuration this is an adjustable voltage divider, which divides the total resistance of the respective voltage divider into two individual resistances by changing the adjustment, thereby dividing an existing total voltage into manageable individual voltages. In audio signal sources containing two audio signal channels for the stereophonic transmission of sound events, and a fader control connected downstream of them, an adjustable voltage divider is provided for each audio signal channel. These two voltage dividers, which have total resistances of equal size, are connected to each other in such a way, that an equal proportion is provided to the respective individual resistances for each voltage divider, in accordance with the adjustment of the fader control. If the audio output signals that exist in the two audio signal channels pass through such a fader control, two audio signals with equal oscillations can be obtained from each audio output channel. The amplitude of the audio signals with equal oscillations can either be the same or of different size, depending on the ratio of the respective individual resistances to each other. Such a fader control can be used for example to divide a two-channel audio in a motor vehicle between two front and two rear loudspeakers, i.e. an audio that can be heard between two--for example front--loudspeakers, can also be heard between two other loudspeakers. Depending on the configuration, the fader control can be an integral component of the audio signal source, perhaps in the form of a car radio, or it can be positioned away from the audio signal source, perhaps on the vehicle's center console.
If the audio signal source contains such a fader control, and if the audio signals are routed to a device for the digital processing of audio signals, depending on the number of output channels in the fader control, four high quality i.e. audio-signal-suitable A/D (Analog/Digital) converters are required to route the audio signals, which were "divided" by the fader control, to a digital processor. A further consequence is that the digital signal processing must also be performed in four channels.
However, since the fader control only has the function of dividing the two-channel sound information between two loudspeaker groups (front and rear loudspeaker groups), it is desirable to subject only the audio output signals that exist in both audio signal channels to digital audio processing, and only then to perform the division for the two loudspeaker groups. Although such a construction has the advantage that only two audio-signal-suitable A/D converters are required, it must be seen as a disadvantage that a fader control of the kind that is integrated in many audio signal sources cannot be used to distribute the sound information to the two loudspeaker groups, but that a separate fader control must be used. In addition, there is often no room for a digital audio signal processor near the audio signal source, so that a number of sometimes lengthy connection lines is necessary between the separate fader control and the device for the digital processing of audio signals, if the separate fader control must be positioned away from the device for the digital processing of audio signals, perhaps in the vicinity of the audio signal source.
The latter applies to both the case where a separate fader control is one as already explained in detail above, as well as for the case where the fader control function is taken over by another regulating device which, depending on the adjustment, directly affects the processors of the device for the digital processing of audio signals.
If the audio signal source is equipped with an integrated fader control, and if the digital signal processing takes place in two channels using an additional fader control, additional measures must be taken to ensure that the audio output signals are available in two-channels before the fader control, independently of the adjustment of the fader control connected to the audio signal source, for the audio signals to be processed by the digital device. Since it cannot be dismissed that a service person may leave the adjustment of the fader control in the audio signal source unchanged, this can only be realized by creating two more outputs in the audio signal source from which the audio output signals can be obtained, regardless of the adjustment of the fader control connected to the audio signal source.